The present disclosure relates generally to difunctional end-capped oligomers, and more particularly to oligomers with di-phenylethynyl endcaps.
Only a few of the thermosetting resins that are commonly used today in fiber-reinforced composites generally can be used in high-temperature applications. These high-temperature thermosetting resins are undesirable in many applications because they often form brittle composites that have relatively low thermal stabilities.
Recently, chemists have sought to synthesize oligomers for high-performance, high-temperature advanced composites suitable for aerospace applications. Most formulations for high-temperature polymer-matrix composites have monofunctional endcaps which limit the degree of crosslinking that can be attained. There is a need for high-performance composites to exhibit solvent resistance, be tough, impact resistant, strong, and be easy to process. There is also a need for oligomers and composites that have thermo-oxidative stability, and can be used at elevated temperatures for extended periods of time.
While epoxy-based composites are suitable for many applications, they are inadequate for applications which require thermally stable, tough composites that are expected to survive for a long time in a hot, oxidizing environment. Recent research has focused on polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness for these high-performance applications. Still the maximum temperatures for use of the polyimide composites, such as those formed from PMR-15, can only be used at temperatures below about 600-625° F. (315-330° C.), since they have glass-transition temperatures of about 690° F. (365° C.). PMR composites may be usable in long-term service (50,000 hours) at about 350° F. (170° C.). They can withstand temperatures up to about 600° F. (315° C.) for up to about five hundred hours.
PMR-15 prepregs, however, suffer significant processing limitations that hinder their adoption because the prepreg has a mixture of the unreacted monomer reactants on the fiber-reinforcing fabric, making them sensitive to changes in temperature, moisture, and other storage conditions, which cause the prepregs to be at different stages of cure. Aging these PMR prepregs even in controlled environments can lead to problems. The reactants on the prepreg are slowed in their reaction by keeping them cold, but the quality of the prepreg depends on its absolute age and on its prior storage and handling history. And, the quality of the composite is directly proportional to the quality of the prepregs. In addition, for some formulations like PMR-15, the PMR monomers may be toxic or hazardous (especially methylendianiline or MDA in PMR-15), presenting health and safety concerns for the workforce. Achieving complete formation of stable imide rings in the PMR composites remains an issue. These and other problems plague PMR-15 composites.
The commercial long-chain polyimides also present significant processing problems. AVIMID-N and AVIMID-KIII (trademarks of E. I. duPont de Nemours) resins and prepregs differ from PMR-15 because they do not include aliphatic chain terminators which PMR-15 uses to control molecular weight and to retain solubility of the PMR-15 intermediates during consolidation and cure. Lacking the chain terminators, the AVIMIDs can chain-extend to appreciable molecular weights. To achieve these molecular weights, however, the AVIMIDs (and their LaRC cousins) rely on the melting of crystalline powders to retain solubility or, at least, to permit processing. It has proven difficult to use the AVIMIDs in aerospace parts because of their crystalline melt intermediate stage.
Imides and many other resin backbones have shown surprisingly high glass-transition temperatures, reasonable processing parameters and constraints for the prepregs, and desirable physical properties for the composites by using soluble oligomers having difunctional caps, especially those with nadic caps. Linear oligomers of this type include two crosslinking functionalities at each end of the resin chain to promote crosslinking upon curing. Linear oligomers are “monofunctional” when they have one crosslinking functionality at each end. Most formulations for high-temperature polymer-matrix composites (HTMPCS) have monofunctional endcaps with the exception of chemistries that contain dinadic endcaps described in U.S. Pat. No. 5,969,079.
It is known that dinadic- and nadic-endcapped materials react at lower temperatures than phenylethynyl-endcapped materials, which can limit some of the chemistries that are possible for HTPMC formulations because they will react too rapidly at the point of minimum viscosity, thereby reducing the available processing window for fabricating parts where liquid-molding processes such as resin transfer molding, resin film infusion, and vacuum-assisted resin transfer molding are desired.
All currently available phenylethynyl-endcapped materials have monofunctional endcaps. Thus, the degree of crosslinking is limited. It has been shown that difunctional endcaps provide polymers with significantly higher mechanical properties than those with monofunctional endcaps, particularly in aerospace-grade epoxies. It would be desirable to provide phenylethynyl-endcapped materials having this multi-functionality for use in new, more-processable materials suitable for high-temperature composites.